1. Field of the Invention
Over 1 billion bushesl of soybeans are processed domestically each year for recovery of soybean oil. Considerable effort has been made to expand world markets for the 24 million tons of defatted soybean meal and soybean flake byproducts produced by the oil extraction industry. Traditonally, the materials have been utilized as ingredients in animal feeds or further processed to produce food-grade protein concentrates or isolates designed for use in protein-supplemented blends for human consumption. One of the principal problems in developing soybean fractions as finished vegetable protein products for human use relates to their inherent flavor. Conventional solvent extraction methods using hexane or similar hydrocarbons leave constituents in the meal which are responsible for raw grassy and bitter flavors. Moreover, residual lipids in the meal tend to auto or enzymtically oxidize into compounds which contribute to development of off-flavors. The result is an eventual reduction in the product's organoleptic and nutritional qualities. Lipoxygenase is the enzyme implicated in part of the oxidative deterioration. Lipoxygenase can be inactivated by heat, but this process has proven unsatisfactory because heat (toasting) reduces the protein solubility as measured by nitrogen solubility index (NSI) from an initial value for the raw bean of 85-90 to unacceptable levels below 60. Alternative solvents such as ethanol and isopropanol have been investigated, but are not competitive with hexane from the standpoint of either solvent properties or recovery. This invention relates to a method of processing soybeans so as to extract the oil and simultaneously produce high-quality, food-grade protein fractions.
2. Description of the Prior Art
In the art of vegetable oil extraction, increasing attention has been given to the technology of supercritical fluids as an alternative to current extraction methods. Supercritical fluids (SCF's) are often referred to as dense gases. Technically, an SCF is a gas existing above its critical temperature and critical pressure, as defined in the phase diagram of the pure substance. When a gas is compressed above its critical temperature, densities increase dramatically. Therefore, under a given set of conditions, an SCF may possess the density of a liquid while maintaining the diffusivity of a gas. Of the several SCF's investigated, supercritical carbon dioxide (SC-CO.sub.2) is ideal because it is nontoxic, nonflammable, nonexplosive, inexpensive, readily available, and easily removed from the extracted products. The solvent properties of SCF's have been recognized for over 100 years, but commercial applications have been slow in developing. Decaffeination of coffee and the extraction of hops with SC-CO.sub.2 are the only current large-scale commercial SCF processes, though other uses are being investigated. British Patent No. 1,356,749 teaches the SCF extraction of oils from crushed or coarsely ground seeds including copra, sunflower, coconut, soybeans, and peanuts. Stahl et al. (J. Agric. Food Chem., 28(6): 1153-1157 (1980) elaborates on the parameters influencing the removal and fractionation of soybean, sunflower seed, and rapeseed oils in terms of yield, color, taste, and odor.
Friedrich et al. (JAOCS 59 (7): 288-292 (July 1982) compares the flavor of hexane-extracted soybean oil to that extracted by SC--CO.sub.2 at pressures of 8000 p.s.i.g. (552 bar) and 50.degree. C. As exemplified by these and other teachings in the art, applications of SCF technology have emphasized the oil recovery, with little or no attention being given to either the flavor or the nitritional value of the seed residue.